Demands imposed on large scale integrated electronic circuits are constantly increasing. Particularly, in the case of electronic data memories, programmable logic modules, and microprocessors, it is important to continue the progress of integration. To ensure the economic success of such devices it is often essential to maximize the number of functional electronic elements on a given substrate die. In the case of electronic data memories, ongoing development is aimed mainly at information density, access speed, and the so-called volatility, the latter being a figure of how long the electronic data memory may reliably hold a stored information content without the need of an external supply of energy.
Whereas volatile memories, such as a DRAM (Dynamic Random Access Memory), store information only for a short time, and, therefore, have to be continuously refreshed, the semiconductor industry has also developed a range of non-volatile memories, such as the Flash RAM. Although a Flash RAM reliably retains the information stored in it for several years without an external energy supply, a large amount of energy is required to write information into a Flash RAM and the integration of a Flash RAM is rather limited due to the respective memory cell's large size.
As a result, substantial scientific and industrial research effort is made to develop new concepts for non-volatile memories. A prominent example of a non-volatile memory is an electronic data memory with programmable resistance cells. These programmable resistance cells change their electric resistance by means of the application of electric signals, while the electric resistance remains stable in the absence of any signals. In this way, such a memory cell may store two or more logic states by a suitable programming of its electric resistance. A binary coded memory cell may then, for example, store an information state “0” via assuming a high-resistive state, and an opposite information state “1” via assuming a low-resistive state.
A material system for such programmable resistance cells are the so-called solid electrolytes, which are already subject to intense research and development. This material system is therefore already well understood as a feasible system for the realization of programmable resistance cells. In materials of this type, a conductive path may be formed from an active electrode material by means of the application of electric signals. Ions from the active electrode material are mobile within the ion-conducting solid electrolyte and can therefore be driven by an electric field into and within the electrolyte. If a path of ions is formed, this path may short-circuit the otherwise high-resistive solid electrolyte between two electrodes, hence drastically reducing the effective electric resistance. Said electrodes may also serve for the application of the electric signals. By reversing the polarity of the electric signal, it is possible to decompose the path of ions such to lead back the programmable resistance cell to a high-resistive state. A so-called inert electrode, which consists of a material that does not dissolve in the electrolyte, often serves as a counter-electrode to the active electrode made from active electrode material.
In this context, the integration of solid electrolyte materials into existing and established fabrication processes for the large scale integrated manufacturing of electronic circuits is of great interest. The most prominent of such manufacturing processes is the so-called CMOS process, which is employed to routinely manufacture highly integrated electronic circuits. Such a CMOS process often comprises several hundred individual process steps and forms an integrated circuit device by means of lithography, deposition, and etching techniques.
Since the required materials for solid electrolyte systems, such as germanium, selenium, silver, or copper, are not part of present established CMOS manufacturing processes, there is a need to introduce methods for a reliable and reproducible handling of the abovementioned materials by a CMOS process. Unfortunately, certain material incompatibilities impose critical obstacles for such a handling, for example when using germanium selenide and silver: silver, as an active electrode material, only grows in thin films in an inhomogeneous form on a solid electrolyte. However, an inhomogeneous electrode layer, in some cases even comprising coagulated islands, may lead to difficulties and problems during further processing and structuring of other device elements and layers. Conventional methods employ an increased layer thickness of the active electrode in order to avoid a disadvantageous inhomogeneous form thereof. This however, in turn, counteracts the objective of an increased integration and a higher packing density of the programmable resistance cells.